Ion mobility spectrometers (IMSs) are used to measure the electrical mobility of charged molecules or particles. Currently two main types of IMS devices exist: crossflow-type and drift tube-type.
Crossflow-type IMSs are commonly used in aerosol science, the most common of which are differential mobility analyzers (DMAs). In DMAs, ions or charged particles are introduced into a stream of gas between two parallel plates or concentric cylinders, across which a voltage differential is applied. The ions or charged particles are introduced through an inlet opening at one of the plates and migrate to the opposing plate following different trajectories according to their electrical mobility, which is a function of the drag coefficient. Particles within a given mobility range exit the DMA via a slit in the opposing plate. The voltage differential, carrier gas flow, exit slit size and location, etc. may be varied to select the electrical mobility range of particles that exit the DMA. The particles exiting the DMA may be directed to an ion detector or other instrument for further analysis.
The inlet plate or cylinder of a DMA is typically grounded, while the opposing plate or cylinder is typically at a negative or positive voltage. Thus, charged particles may readily enter the inlet of a DMA because there is no voltage potential to overcome. Accordingly, samples containing ionized species such as ambient environmental aerosols may be readily introduced into DMAs.
DMAs, however, have some drawbacks. First, the carrier gas flow rate tends to be quite high in order to effectively separate smaller charged particles, such as particles within the 1-10 nm range. In addition, DMAs tend to have a low resolving power for such smaller particles.
Drift tube-type IMSs (DT-IMSs) do not tend to suffer from the drawbacks of DMAs, which include low resolving power for small particles, high gas flow rates, and long measurement times. However, DT-IMSs present obstacles for aerosol measurements or sampling of particles that are charged prior to entering the DT-IMS. This feature is important for sampling aerosols where the user does not wish to alter the charge distribution or in cases where a charged aerosol has been selected by an upstream tandem device. Another obstacle of DT-IMSs for aerosol measurements is that DT-IMSs typically employ ion detectors having a low sensitivity. Ambient aerosols have ion concentrations that are often several orders of magnitude lower than molecular ion concentrations traditionally measured with DT-IMSs.
As indicated above, DT-IMSs are typically used for measurement of molecules or particles that are not initially charged. The uncharged molecules or particles enter a chamber across which an elevated voltage is applied. As the molecules are uncharged, their entry into the chamber is not significantly impeded by the voltage difference. However, particles that are charged or ionized prior to entering the chamber cannot readily cross the voltage barrier and thus cannot readily enter the chamber for analysis. Sampling of charged particles by a common DTIMS is possible through the use of a dielectric sampling port although the losses of charged particles would likely be high.
A schematic diagram that illustrates a generic DT-IMS 10 is shown in FIG. 1, with the voltage differential across the device shown. The molecules or particles to be analyzed are introduced through inlet 120 to enter a chamber 105 or portion of the drift tube 200 held at an elevated voltage. While in the chamber 105, an ionization source 165 is employed to ionize the particles. Once ionized the particles migrate in the drift tube 200 down the voltage differential, which is applied by a plurality of axially aligned conductive drift rings (not shown). An electronic gate 100 is typically employed to precipitate the particles until the gate is opened. The gate 100 is opened and then closed allowing a packet of particles to enter the drift zone 202, in which particles migrate towards a detector 150 down the voltage differential. A carrier gas is introduced through inlet 140 parallel to, but in opposing direction of, the direction of particle migration to purge the measurement region of unionized species. Particles reaching the ion detector 150 at various times after the opening of the gate 100 are detected, and data regarding the electrical mobility of the particle (which is proportional to the drift time) may be collected or analyzed. As shown in FIG. 1, the device 10 may include an outlet 130 for the carrier gas and may include an aperture grid 110 upstream of the detector 150. Most commonly, the detector 150 of a DT-IMS 10 is a Faraday plate, which tends to be insufficiently sensitive for the detection of ions or charged particles in concentrations typically found in aerosols.